Some theories, most notably special and general relativity, suggest that suitable geometries of spacetime, or certain types of motion in space, may allow time travel into the past and future if these geometries or motions are possible. Concepts that aid such understanding include the closed timelike curve.
Albert Einstein's special theory of relativity (and, by extension, the general theory) very explicitly permits a kind of time dilation that would ordinarily be called time travel. The theory holds that, relative to a stationary observer, time appears to pass more slowly for faster-moving bodies: for example, a moving clock will appear to run slow; as a clock approaches the speed of light its hands will appear to nearly stop moving. The effects of this sort of time dilation are discussed further in the popular "twin paradox".
A second, similar type of time travel is permitted by general relativity, where a distant observer sees time passing more slowly for a clock at the bottom of a deep gravity well, and a clock lowered into a deep gravity well and pulled back up will indicate that less time has passed compared to a stationary clock that stayed with the distant observer.
These effects are to some degree similar to hibernation or hypothetical suspended animation (which slow down the rates of chemical processes in the subject), and only allow "time travel" only toward the future: never backward. They do not violate causality. This is not typical of the "time travel" featured in science fiction (where causality is violated at will), and there is little doubt surrounding its existence. "Time travel" will hereafter refer to travel with some degree of freedom into the past or future of proper time.
Many in the scientific community believe that time travel is highly unlikely. This belief is largely due to Occam's Razor. Any theory which would allow time travel would require that issues of causality be resolved. What happens if you try to go back in time and kill your grandfather?—see grandfather paradox. Also, in the absence of any experimental evidence that time travel exists, it is theoretically simpler to assume that it does not happen. Indeed, Stephen Hawking once suggested that the absence of tourists from the future constitutes a strong argument against the existence of time travel—a variant of the Fermi paradox, with time travelers instead of alien visitors. However, assuming that time travel cannot happen is also interesting to physicists because it opens up the question of why and what physical laws exist to prevent time travel from occurring.
Time traveling
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The "presentist" view
Some theorists have argued that the matter of the universe only exists in the present moment. Thus, if one were to travel back from the 'present' to an earlier time, none of the material universe would be found there, because it will have remained in the present: the traveller alone is the only part of the universe to have gone back to the earlier time. In terms of a 4-dimensional spacetime, the traveller (or, more generally the atomic particles that comprise the traveller) would have travelled 'back' to an area of spacetime corresponding to an earlier value of 't'; but none of the other particles that form the universe will have done so, so the traveller finds precisely nothing when arriving back at the earlier time. This viewpoint eliminates all of the supposed paradoxes about time travel.
The equivalence of time travel and faster-than-light travel
If one were able to move information or matter from one point to another faster than light, then according to special relativity, there would be an observer who sees this transfer as allowing information or matter to travel into the past. Additionally, faster than light travel along suitable paths would correspond to travel backward in time as seen by all observers. This results simply from the geometry of spacetime and the role of the speed of light in that geometry.
Special spacetime geometries
The general theory of relativity extends the special theory to cover gravity, describing it in terms of curvature in spacetime caused by mass-energy and the flow of momentum. General relativity describes the universe under a system of "field equations," and there exist solutions to these equations that permit what are called "closed time-like curves," and hence time travel into the past. The first and most famous of these was proposed by Kurt Gödel, but all known current examples require the universe to have physical characteristics that it does not appear to have. Whether general relativity forbids closed time-like curves for all realistic conditions is unknown. Most physicists believe that it does, largely because assuming some principle against time travel prevents paradoxical situations from occurring.
Using wormholes
A proposed time-travel machine using a wormhole would (hypothetically) work something like this: A wormhole is created somehow. One end of the wormhole is accelerated to nearly the speed of light, perhaps with an advanced spaceship, and then brought back to the point of origin. Due to time dilation, the accelerated end of the wormhole has now experienced less subjective passage of time than the stationary end. An object that goes into the stationary end would come out of the other end in the past relative to the time when it enters. One significant limitation of such a time machine is that it is only possible to go as far back in time as the initial creation of the machine; in essence, it is more of a path through time than it is a device that itself moves through time, and it would not allow the technology itself to be moved backwards in time. This could provide an alternative explanation for Hawking's observation: a time machine will be built someday, but has not yet been built, so the tourists from the future cannot reach this far back in time.
According to current theories on the nature of wormholes, creating a wormhole of a size useful for a person or spacecraft, keeping it stable, and moving one end of it around would require significant energy, many orders of magnitude more than the Sun can produce in its lifetime. Construction of a wormhole would also require the existence of a substance known as "exotic matter", which, while not known to be impossible, is also not known to exist in forms useful for wormhole construction (but see for example the Casimir effect). Therefore it is unlikely such a device will ever be constructed, even with highly advanced technology. On the other hand, microscopic wormholes could still be useful for sending information back in time.
Matt Visser argued in 1993 that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely than not a flaw in classical quantum gravity theory rather than proof that causality violation is possible.
Another approach — attributed to Frank Tipler, but invented independently by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924 — involves a spinning cylinder. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string.
Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time. However, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.
According to current theories on the nature of wormholes, creating a wormhole of a size useful for a person or spacecraft, keeping it stable, and moving one end of it around would require significant energy, many orders of magnitude more than the Sun can produce in its lifetime. Construction of a wormhole would also require the existence of a substance known as "exotic matter", which, while not known to be impossible, is also not known to exist in forms useful for wormhole construction (but see for example the Casimir effect). Therefore it is unlikely such a device will ever be constructed, even with highly advanced technology. On the other hand, microscopic wormholes could still be useful for sending information back in time.
Matt Visser argued in 1993 that the two mouths of a wormhole with such an induced clock difference could not be brought together without inducing quantum field and gravitational effects that would either make the wormhole collapse or the two mouths repel each other. Because of this, the two mouths could not be brought close enough for causality violation to take place. However, in a 1997 paper, Visser hypothesized that a complex "Roman ring" (named after Tom Roman) configuration of an N number of wormholes arranged in a symmetric polygon could still act as a time machine, although he concludes that this is more likely than not a flaw in classical quantum gravity theory rather than proof that causality violation is possible.
Another approach — attributed to Frank Tipler, but invented independently by Willem Jacob van Stockum in 1936 and Kornel Lanczos in 1924 — involves a spinning cylinder. If a cylinder is long, and dense, and spins fast enough about its long axis, then a spaceship flying around the cylinder on a spiral path could travel back in time (or forward, depending on the direction of its spiral). However, the density and speed required is so great that ordinary matter is not strong enough to construct it. A similar device might be built from a cosmic string, but none are known to exist, and it does not seem to be possible to create a new cosmic string.
Physicist Robert Forward noted that a naïve application of general relativity to quantum mechanics suggests another way to build a time machine. A heavy atomic nucleus in a strong magnetic field would elongate into a cylinder, whose density and "spin" are enough to build a time machine. Gamma rays projected at it might allow information (not matter) to be sent back in time. However, he pointed out that until we have a single theory combining relativity and quantum mechanics, we will have no idea whether such speculations are nonsense.
Using Quantum Entanglement
Quantum-mechanical phenomena such as quantum teleportation, the EPR paradox, or quantum entanglement might appear to create a mechanism that allows for faster-than-light (FTL) communication or time travel, and in fact some interpretations of quantum mechanics such as the Bohm interpretation presumes that some information is being exchanged between particles instantaneously in order to maintain correlations between particles. This effect was referred to as "spooky action at a distance" by Einstein.
Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. The fact that these quantum phenomena apparently do not allow FTL/time travel is often overlooked in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research.
Nevertheless, the rules of quantum mechanics curiously appear to prevent an outsider from using these methods to actually transmit useful information, and therefore do not appear to allow for time travel or FTL communication. The fact that these quantum phenomena apparently do not allow FTL/time travel is often overlooked in popular press coverage of quantum teleportation experiments. The assumption that time travel or superluminal communications is impossible allows one to derive interesting results such as the no cloning theorem, and how the rules of quantum mechanics work to preserve causality is an active area of research.
The possibility of paradoxes
The Novikov self-consistency principle and recent calculations by Kip S. Thorne indicate that simple masses passing through time travel wormholes could never engender paradoxes—there are no initial conditions that lead to paradox once time travel is introduced. If his results can be generalised, they would suggest, curiously, that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, that any situation you can set up in a time travel story turns out to permit many consistent solutions. The circumstances might, however, turn out to be almost unbelievably strange.
Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction.
In a 2005 paper, Professor Daniel Greenberger of City University of New York and Karl Svozil of the Vienna University of Technology proposed that quantum theory gives a model for time travel without paradoxes. [5] In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.
Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible (see Chronology protection conjecture).
A further suggestion related to paradoxes suggests that time travel will never exist, even if theoretically possible. The reasoning is that as long as time travel exists, history will change, and will only become static when a timeline is reached in which no time travel exists and thus no further changes can be made. Assuming there is only a single dimension of time, the timeline we perceive must be the one that exists after all changes (if any) are made, and thus we will never perceive the invention of time travel, since it will have already destabilised itself out of the timeline by the time we would have reached it.
Parallel universes might provide a way out of paradoxes. Everett's many-worlds interpretation of quantum mechanics suggests that all possible quantum events can occur in mutually exclusive histories. These alternate, or parallel, histories would form a branching tree symbolizing all possible outcomes of any interaction.
In a 2005 paper, Professor Daniel Greenberger of City University of New York and Karl Svozil of the Vienna University of Technology proposed that quantum theory gives a model for time travel without paradoxes. [5] In quantum theory observation causes possible states to 'collapse' into one measured state; hence, the past observed from the present is deterministic (it has only one possible state), but the present observed from the past has many possible states until our actions cause it to collapse into one state. Our actions will then be seen to have been inevitable.
Since all possibilities exist, any paradoxes can be explained by having the paradoxical events happening in a different universe. This concept is most often used in science-fiction. However, in actuality, physicists believe that such interaction or interference between these histories is not possible (see Chronology protection conjecture).
A further suggestion related to paradoxes suggests that time travel will never exist, even if theoretically possible. The reasoning is that as long as time travel exists, history will change, and will only become static when a timeline is reached in which no time travel exists and thus no further changes can be made. Assuming there is only a single dimension of time, the timeline we perceive must be the one that exists after all changes (if any) are made, and thus we will never perceive the invention of time travel, since it will have already destabilised itself out of the timeline by the time we would have reached it.
Time travel and the direction of time
The notion of time travel (either towards the future or towards the past) tacitly assumes that there exists a direction of time, the direction from the past to the future. On the other hand, the direction of time (or the arrow of time) may not be a fundamental intrinsic property of time, but rather could be viewed as an emergent property traceable to the fact that we live in a universe in which the entropy increases with time. In this view, as the direction of time is not fundamental, the notion of time travel is also not fundamental. Without a fundamental notion of time travel there can be no fundamental problems with time travel. Without an intrinsic direction of time, time can be viewed as a "static" coordinate similar to other spacetime coordinates. From this point of view, the Novikov self-consistency principle is a tautology, a demand that hardly needs to be questioned, which automatically prevents causal paradoxes.
Time travel and the anthropic principle
It has been suggested by physicists such as Max Tegmark that the absence of time travel and the existence of causality may be due to the anthropic principle. The argument is that a universe which allows for time travel and closed time-like loops is one in which intelligence could not evolve because it would be impossible for a being to sort events into a past and future or to make predictions or comprehend the world around them.
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